few choices depends how far you wanna go.........

1) damp proof paint - works but just covers the problem
2) take all plaster off to 1 metre high (rising damp only tracks up 1 metre from ground level usually) and replaster using a waterproofer mixed in)
3) Take plaster off as above, drill 10mm holes every 6 inch in motar joints and fill with damp proofing compound (holes to be 1st coarse abouve ground level inside) then follow as per No2